1. Technical Field
The present disclosure relates to an apparatus for auto focus for a camera module, and more particularly, to an apparatus for auto focus with a three-location supporting structure, which has an improved ball supporting structure for the movement in an optical direction.
2. Background Art
Along with the development of hardware technique for image processing and the increase of user needs for image photographing, an auto focus (AF) function, an optical image stabilization (OIS) function or the like is implemented in a camera module mounted to a mobile terminal such as a cellular phone and a smart phone as well as a camera.
The auto focus (automatic focusing) function adjusts a focal distance to a subject by linearly moving a lens or an assembly having the lens in an optic-axial direction so that a clear image is generated at an image sensor (CMOS, CCD or the like) provided at a rear end of the lens.
In order to implement the auto focus function, various methods are used. Representatively, a magnet (a permanent magnet) is installed at an AF carrier (or, a moving body), a coil is installed at a fixed body (a housing, or another-type carrier or the like), and a power of a suitable intensity is applied to the coil to generate an electromagnetic force at the coil (provided at the fixed body) and the magnet (provided at the moving body) so that the moving body moves in an optic-axial direction.
In addition, a device or actuator in which the AF and OIS functions are integrated has been recently used. In this case, a structure for moving an OIS carrier (or, a frame, a lens assembly or the like) having a lens loaded therein within the AF carrier in a direction perpendicular to the optic-axial direction is integrally implemented together with the AF structure described above. As another implementation, a lens may be located at the AF carrier, and an OIS carrier provided out of the AF carrier may move in a direction perpendicular to the optic-axial direction.
Meanwhile, in an existing device where only the AF function is implemented or the AF and OIS functions are implemented together, as shown in FIG. 1, balls 510-1, 510-2 arranged in the same direction as an optical axis are interposed between an AF carrier (a moving body) 500 and a housing (a fixed body) (not shown) in order to improve the behavior characteristic of the AF carrier 500 moving in the optic-axial direction.
In this structure, a suitable distance between the moving body and the fixed body may be consistently maintained, and a friction is minimized by means of rotating motions of the balls and point contact of the balls, so that the AF carrier may move in the optic-axial direction more flexibly and more accurately.
In the existing technique, a plurality of balls b1 to b6 having the same size (diameter) d1 to d6 are used, or balls having the same size are used but four balls having a greater size are disposed outer sides so that the four balls give a support. In this case, theoretically, all balls (or, four balls) make point contact simultaneously so that the horizontal direction of the AF carrier is maintained in operation. However, actually, the balls do not make point contact simultaneously, and thus a fault occurs at the horizontal tilt of the AF carrier.
In detail, first, sizes of balls cannot be physically perfectly identical, and thus ideal sameness cannot be implemented. Thus, since contacts are complexly made by a plurality of objects in this structure, rather than by a single object, a physical gap may occur, which generates a resultant tilt fault.
In addition, since the AF carrier is not always fixed but repeats moving in an optic-axial direction and stopping, static frictions and kinetic frictions of differential intensities are generated, and such differential frictions cause a gap. Thus, it is actually impossible for all balls to make point contact simultaneously, and thus a tilt fault of the AF carrier tilt fault occurs.
Further, even though a ball is adhered to one side of the AF carrier due to an attractive force generated between the magnet of the AF carrier and the yoke provided at the fixed body, the AF carrier extending in a horizontal direction is more influenced by gravity at a region farther from the side adhered to the ball, namely as the degree of extension increases. This also makes it impossible that all balls make point contact simultaneously. As a result, the above factors are complexly applied to generate a tilt fault of the AF carrier.
In the existing technique, a plurality of balls are simply disposed without any consideration of the above problems. Thus, due to the above problems, in an existing device, when an AF carrier 500 moves in an optic-axial direction, balls making point contact with the AF carrier are frequently changed, and such frequent change of balls making point contact resultantly collapses the balance of the AF carrier 500, thereby causing tilt faults θ1 and θ2 of the AF carrier 500.
Such tilt faults deform a path of light introduced into an image sensor 600 through a lens as much as a maximum separation angle (θ=θ1+θ2), which causes an error in focus adjustment as much and accordingly causes problems in generating a clear image. Recently, a camera module loaded in a smart phone or the like is implemented with a light and slim design. In this slim design, a ratio of thickness to width of the AF carrier becomes greater, and thus the tilt problem of the AF carrier as above becomes worse.